The present invention relates to a slide driving device for presses. In particular, the present invention relates to a slide driving device for presses that convert energy from a hydraulic fluid into a drive force that is applied to a slide driving mechanism in a press.
Conventional slide driving devices for presses include mechanical devices in which energy is accumulated in a flywheel driven by an electric motor. This energy is transferred to a slide via a crank shaft thus providing efficient and high-cycle continuous operations. Alternatively hydraulic slide driving devices which use a hydraulic fluid to drive a slide can be used. Another type of slide driving device is the AC servo device. In this device a screw mechanism serves as a slide driving mechanism and this screw mechanism drives an AC servo motor. Each of these types of conventional slide driving devices for presses has advantages and disadvantages in the areas of energy efficiency, controllability, down-sizing, and the like.
Referring to FIG. 20 there has been developed a slide driving device for presses (Japanese Laid-Open Publication Number 1-309797) that drives a crank shaft using a hydraulic motor and a variable flow discharge pump. The object of this technology is to combine the high-cycle properties of the mechanical method described above with the ability to perform variable speed control provided by the hydraulic method described above.
Referring to FIG. 20 the slide drive device for presses includes a variable displacement pump 5 which receives a drive force from a motor 1 via a flywheel 2 a clutch brake 3 and a decelerator 4. A variable displacement motor 6 is rotated according to the flow discharged from variable displacement pump 5. Variable displacement motor 6, in turn, rotates a crank shaft 8 of a crank press 7. A control device 9, illustrated as a central processing unit (CPU), receives as inputs the rotation speed and the swash plate angle of variable displacement pump 5 and the rotation speed of crank shaft 8. An output of control device 9 controls the swash plate angle of variable displacement motor 6 and/or variable displacement pump in a manner to control the speed of a controlled slide to a pre-set slide speed.
Referring to FIG. 21(a) there is shown a schematic drawing of the slide driving device for presses. Referring to FIG. 21(b) there is shown a schematic block diagram of the device shown in FIG. 21(a) Referring to FIG. 21(c) there is shown a redrawn version of FIG. 21(b).
The following are the symbols used in the drawings and their meanings.
J: moment of inertia (kg cm.sup.2) PA1 q: displacement volume (cm.sup.3 /rad) PA1 Q: oil flow (cm.sup.3 /s) PA1 K: oil's bulk modulus of elasticity (kg/cm.sup.2) PA1 g: acceleration of gravity (cm/s.sup.2) PA1 s: Laplace operator (1/s: integral) PA1 V: volume of pipe system (cm.sup.3) PA1 .OMEGA.: angular velocity (rad/s) PA1 D: viscosity resistance coefficient (kg cm s/rad) PA1 secondary lag={.OMEGA.a.sup.2 /(s.sup.2 +2xi.OMEGA.a s+.OMEGA.a.sup.2)} PA1 where .OMEGA.a.sup.2 =q.sup.2 gK/(2.pi.V J) PA1 xi=(D/Q)*{(.pi.g V)/(2KJ)}.sup.(1/2).
Referring to FIG. 21(c) in a static state oil flow Q can be expressed as Q=.OMEGA.*q/(2.pi.). Displacement velocity q is proportional to angular velocity .OMEGA..
In a dynamic state the second-order lag expressed in the equation below takes place from the given oil flow Q until the required torque at the commanded angular velocity of the rotation of the hydraulic motor is generated:
The conventional slide driving device for presses described above provides control of the oil flow for the hydraulic motor. The rotation speed of the hydraulic motor is determined by the oil flow supplied to the hydraulic motor. Thus a large amount of hydraulic fluid is required. The amount of hydraulic fluid is proportional to the product of the rotation speed and the displacement volume. As a result the oil-pressure generating device, the pipe capacity, and the like, must be large.
Also the torque required to drive the hydraulic motor is the product of the displacement volume and the pressure generated by compression of the hydraulic fluid in the pipe system. As described above, assuming ideal conditions, a secondary lag (90 degree phase delay in the natural frequency) is generated up to the point when the given oil flow results in a commanded angular velocity. In practice this characteristic is the dominant tendency. Thus a high degree of precision in control cannot be attained in system speed (responsiveness) and the like.